Roof Top Gardens: An Overview
56A – October 2005
Roof Top Gardens are becoming increasingly popular as building owners and managers strive to make new and existing buildings more environmentally advanced, energy efficient and sustainable. Building owners and designers should be aware that the installed cost of these systems is significantly higher than conventional roofing systems. The design, material and installation requirements of green roofs must be fully understood to achieve optimal performance throughout their service lives. This bulletin is the first in a series of Roof Top Gardens. These bulletins will address and are intended to address construction and design issues relating to roof top garden construction.
There are several terms used to describe these systems including roof top gardens— vegetated roof covers, eco-roofs and even “green roofs”. The roofing industry prefers the former — Roof Top Gardens — as it differentiates between more traditional plaza or podium decks and on grade landscaping and/or roofing systems incorporating thin layers of living vegetation installed on top of conventional flat or sloping roofs. The term green roofs should not be used to describe these systems as it has many meanings and as often used to denote the overall environmental attributes of the roof assembly.
There are many types of roof top gardens varying from relatively lightweight systems with a thin covering of soil to systems that look like ordinary gardens with much thicker layers of soil with a variety of shrubs and trees.
Lightweight systems with a thin cover of soil are called Extensive Systems whereas those with thick soil layers are called Intensive Systems. There is a whole range in between sometimes referred to as Semi-Intensive Systems. The FLL’s (The Landscaping and Landscape Development Research Society) categorizes green roof systems on the basis of the vegetative cover. In North America, we prefer to categorize them on the basis of the load of these systems (when saturated). Although there is no hard and fast rule for the weight of each system, the following table shows the appropriate weight of each system when fully saturated.
The saturated weight of the system will depend on the type of growing medium and the mass of the components. Whichever type of roof top garden system is selected, there are a number of fundamental technical requirements that must be addressed in its design and construction.
Protection from root penetration
The roof membrane system will require protection against root ingress or penetration. Barriers to root penetration should not be limited to just those areas covered with vegetation. Any joints, borders, transitions and roof projections must be adequately treated to prevent root penetration.
Root protection can be provided by a variety of means. It can be incorporated in the drainage layer or membrane materials incorporating or coated with root repelling chemical agents. Some membrane materials such as EPDM or PVC are inherently root resistant. Special laminated membranes (usually with an upper layer of copper) are also available. Another method of providing a root barrier is to install a separate root resistant and inorganic protective sheet. This can be a thick polyethylene, a rubber sheet, or even a thin layer of copper. To be effective, sheet materials must be properly seamed. To prevent abrasion and mechanical damage, a protection layer may be necessary when the sheet is to be applied over rough surfaces. Whenever such root barrier is to be exposed to the elements (i.e., at borders or upstands) it must be UV resistant.
The roofing system may consist of single ply, liquid applied or multi-layer asphaltic systems. The nature of the roofing membrane system will determine the need for a separate root barrier. Correct and meticulous application of the roof membrane is essential for the long-term performance of the system. Quality assurance through rigorous monitoring of the installation and strict adherence to the specifications are required. All roof top garden installations require a water impermeability test immediately after the installation of the membrane and prior to the installation of the above membrane components. It is strongly recommended that a reliable integrated leak detection system be incorporated into the roof system so that the source of any leaks can be quickly located.
The requirement for insulation in roof top garden systems will be dictated by the design criteria. The insulation can be placed either below the membrane (conventional assembly) or above the membrane
(protected membrane assembly). In both configurations, structural strength and resistance to plastic creep (irreversible deformation under load) are critical properties. In PMR assemblies, moisture resistance is required to maintain the necessary structural strength as well as thermal resistivity.
All roof top gardens must be properly drained to maintain optimum growing conditions and manage rainfalls or melting ice and snow. Every green roof must have a drainage layer to carry away excess water to prevent soil erosion or rotting of the vegetation. The drainage layer also acts as a temporary water storage reservoir providing water to the plants under dry conditions. The drainage system must be capable of collecting both overflow from the drainage course and surface water from the vegetation support course and conveying it away. In extensive systems, the drainage layer may be combined with the filter layer.
A filter layer is installed below the growing medium to hold the soil in place and prevent small soil particles and plant debris from entering and clogging the drainage layer below.
Careful placement is required to ensure sufficient overlaps. The filter fabric must be carried up along vertical edges and secured. There are a wide variety of geotextiles designed for use as filter fabrics.
The growing medium or soil substrate can be selected from several engineered factory mixes available from roof garden suppliers, or they can be custom designed by a soil expert. The soil media type and thickness will determine the weight of the assembly as well as its water-absorption capacity. The soil also impacts on how well the plants will establish and how much maintenance, weeding, and fertilization will be necessary.
The weight of green roof soil media is always specified at maximum water retention capacity per unit of thickness (depth). Along with weight considerations, the growing media must meet several other requirements including grain size, soil density, pH, organic, and nutrient content.
The plants that form the vegetative cover, particularly those in shallow systems, must be able to withstand significant exposure to sun, wind, drought, and heavy rains at times. Selection of plants should be based on considerations of the system’s anticipated use and climatic conditions. Key considerations are temperature, humidity, rainfall, sun/shade and wind exposure. The hardiness of the plants (heat, drought and frost resistance) is an important element to consider. Designers must be aware that roof top environment varies significantly from ground level conditions.
Putting it all together
The success of a roof top garden installation requires the careful selection of all the components that comprise the system. Measures must be taken to ensure that the integrity of the waterproofing is maintained throughout the service life of the roof while the vegetative cover flourishes.
The opinions expressed herein are those of the CRCA National Technical Committee. This Technical Bulletin is circulated for the purpose of bringing roofing information to the attention of the reader. The data, commentary, opinions and conclusions, if any, are not intended to provide the reader with conclusive technical advice and the reader should not act only on the roofing information contained in this Technical Bulletin without seeking specific professional, engineering or architectural advice. Neither the CRCA nor any of its officers, directors, members or employees assume any responsibility for any of the roofing information contained herein or the consequences of any interpretation which the reader may take from such information.